The invention relates to a method of manufacturing friction linings.
Compression of the friction linings from a friction material mixture is effected by means of a pressing process comprising at least one pressing cycle performed in a press, wherein a press control unit controls, individually or in combination with each other, a plurality of process parameters as manipulated variables to attain a given lining property of the friction linings. This lining property may e. g. be the compressibility of the friction lining, the density, the moduli of elasticity in the three space coordinates or the dimensions of the friction lining.
Quality deviations of the friction material mixture, wear of tools, temperature changes in the pressing tool as well as tolerances of the press control unit are disturbance variables which may considerably affect the lining property of the friction linings even if all other process parameters remain constant.
It is therefore an object of the invention to provide a method for manufacturing friction linings where malproduction is reduced and the reproducibility of the friction lining properties is increased.
The invention advantageously provides that at least one characteristic value representative for the lining property of the friction linings is measured in a single pressing cycle or in a plurality of pressing cycles and that the process parameters as manipulated variables are controlled in dependence on at least one representative measured characteristic value for the current pressing cycle, for the subsequent pressing cycles and/or subsequent pressing processes. The invention thus alms at measuring the characteristic values during the pressing process and use the thus obtained data for directly controlling the press. Quality deviations of the mixture, wear of tools, temperature change in the pressing tool as well as tolerances of the press control unit can thus be compensated for to a large degree whereby the lining property as a controlled variable can be kept constant. In this manner production variations with regard to the lining properties are minimized and a high reproducibility of the lining quality is attained. The process according to the invention allows improvement of the automation of the production process, in particular in connection with a process control station for a plurality of presses.
As manipulated variables the path travelled by the pressing stamp, the stamping pressure, the pressing and venting times and the pressing temperature can be used individually or in combination with each other. These process parameters can be separately measured on the press and controlled by the press control unit. For example, the stamping pressure, the pressing and venting times and the press temperature can be combinedly readjusted within given limits.
Further, the composition of the friction material mixture, in particular the resin content of the friction material mixture, and/or the quantity of friction material mixture and/or the quantity of a friction material mixture forming the intermediate layer can be used as manipulated variables. In this manner quality deviations of the friction material mixture and/or the weight-in quantity of the friction material mixture can be optimized.
The characteristic value representative of the lining property of the friction linings can, according to a first embodiment of the invention, be obtained from a hysteresis loop of one or a plurality of pressing cycles, said hysteresis loop being supplied by measurement of the stamping pressure in dependence on the path travelled by the pressing stamp.
The measurement of the stamping pressure during opening and closing of the pressing tool in dependence on the path travelled by the pressing stamp results in a hysteresis loop whose characteristic values are directly correlated to the compressed friction lining, in particular to the compressibility of the friction lining. For example, the measured value of the area of the hysteresis loop can be directly used as representative characteristic value for press control purposes.
Alternatively, the representative characteristic value can be obtained from the flow path of the pressing stamp after a given maximum stamping pressure has been reached. When this given stamping pressure has been reached, the further path travelled by the stamp as from this time up to stilstand of the pressing stamp is measured.
According to a further alternative the maximum value of the path travelled by the stamp can be used as representative characteristic value of the lining property of the friction linings.
The representative characteristic value can also be obtained from the ascending slope of the relief curve section of the hysteresis loop supplied by measurement of the stamping pressure in dependence on the path travelled by the pressing stamp.
According to a second embodiment the characteristic value representative of the lining property of the friction lining can be obtained from a hysteresis curve supplied by measurement of the stamping pressure in dependence on the radial pressure acting upon the tool inner wall of the mold.
The characteristic value can e. g. be the area of this hysteresis curve or the maximum value of the radial pressure acting upon the tool inner wall of the mold.
Further, the representative characteristic value can be the pressure difference of the radial pressure acting upon the tool inner wall of the mold when a given maximum stamping pressure has been reached. This pressure difference. correlates well with the compressibility of the friction linings.
Generally, the characteristic value can be the ascending slope value of a predetermined curve section of the hysteresis loop supplied by measurement of the stamping pressure in dependence on the path travelled by the pressing stamp or measurement of the stamping pressure in dependence on the radial pressure acting upon the tool inner wall.
It is possible to control, in terms of time, a predetermined pressure buildup during each pressing cycle by means of measurement of the actual pressing time and measurement of the radial pressure acting upon the tool inner wall.
By measurement of the actual pressing times the aimed pressing times can be exactly adhered to. Form-to-form deviations or press-to-press deviations can be individually compensated for, the same applies to deviations of the pressure buildup and the operating temperature of the press.
The pressure buildup and the pressure relief of the hysteresis curve according to the first and second embodiments can be controlled by time control of the stamping pressure such that the ascending slopes of the pressure buildup curve and the pressure relief curve are almost identical.
By means of temperature measurement the energy consumption of the friction material mixture can be measured, and the temperature measuring signal for controlling the press temperature can be used as manipulated variable.
Alternatively, the radiation heat of the friction lining ejected after the pressing process can be measured, and this temperature measuring signal for controlling the press temperature can be used as manipulated variable.
According to a further alternative, the electrical heating capacity of the press can be measured, wherein this measuring signal controls the press temperature as a manipulated variable.
With regard to the lining properties the compressibility, the density, the moduli of elasticity in the three space coordinates or the dimensions of the friction lining or a combination of the aforementioned friction properties can be used as controlled variable.
The aforementioned method is suited as a method for testing friction lining mixtures. In this manner the reproducibility of the friction material quality can be checked prior to compression, and the friction material mixture can be corrected, if necessary.